I've yet to get around to Game of Thrones. I was initially surprised when Game of Thrones crossed over so much to casual people also but I think casual people like the shocking nature of it and the who's gonna die aspect of it every week. At least that's what they tell me. That isn't a shot at Game of Thrones. I'm sure its a really good show. More what I'm saying is that it feels like Game of Thrones maybe has more spectacle than Expanse although not having seen it, and not wanting to get spoiled, I'm guessing.
My kneejerk reaction is that slow talky sci-fi always struggles crossing over to casuals. That being said, Westworld is pretty slow and talky itself and seems to be popular although the theme isn't space stuff and its on a more prestigious channel like HBO and it has a gratuitous amount of sex and violence baked in to try to lure audience.
I think being on SyFy network also hurts. I don't think casual people take that channel very seriously or are aware of it.
Ultimately I think the slowness, being on SyFy, and the lack of promotion is what killed it. I was vaguely aware of the show when it first launched but honestly completely forgot about it until people started talking about it in this thread.
Also let me be clear that I adore the slowness of it. It's slowness done right. It's slow because its showing multiple sides and building out its story. But I understand how that's a turn off to casual people who always seem to complain anytime crazy shit isn't exploding on the screen every week.